It is common for sensitive electrical apparatus, such as computer disk drives, to be shock mounted within a frame. The shock mounting reduces the possibility of damage from a sudden impact, such as dropping the device onto a hard surface. Some devices require isolation mounts, which not only protect against shocks and other high frequency vibrations, but also against lower frequency vibrations as well. These mounts must additionally be able to cope with internally generated vibrations, for example those due to the access motion of a disk file actuator.
Associated with a shock or isolation mount is a mount frequency, which is the natural frequency of oscillation of the mounted device due to the elastic restoring force of the mounts. For a device driven by input oscillations of a given frequency and amplitude, the ratio of the amplitude of the output response oscillations to that of the input oscillations is referred to as the gain. The general variation of gain with frequency (termed the transfer function) is well known for such systems. For input oscillations at much less than the natural (in our case mount) frequency, the gain is approximately unity--i.e the system behaves as if the mounting were simply a rigid connection. Then, as the driving input frequency becomes comparable with the mount frequency, resonance occurs and the gain becomes large. Finally, above the mount frequency resonance, the gain falls beneath unity, and thereafter the gain continues to decrease with increasing frequency.
In the design of a shock or isolation mount, adjustment of the mount frequency can be used to control vibration at different frequencies. Typically such mounts include some form of damping to further reduce motion, and this can also affect the height, width, and exact position of the mount frequency resonance.
One common type of prior art shock mount is described in PCT application WO 88/09551. In this design, a rubber grommet is placed around a cylindrical projection of the head-disk enclosure (or alternatively around a bolt that is screwed part way into the head-disk enclosure). The grommet is then held within the support frame, to provide a shock mount for the head-disk enclosure. Typically there are three or four such shock mounts connecting the head-disk enclosure to the support frame. A slight variation of this method is disclosed in U.S. Pat. No. 4812932, which describes a grommet with a spring constant that is arranged to vary significantly with displacement. U.S. Pat. No. 4683520 discloses a shock mount which again uses a grommet, but instead of having the grommet held directly by the frame, attaches it to a cantilevered extension of the frame.
A somewhat different mounting system for unspecified electrical equipment is disclosed in Canadian patent 788873. This uses a panel of laminar material including a damping layer as a support structure for apparatus. Resilient grommets are used to attach the panel to a frame, and apparatus to the panel. The use of the panel enhances the isolation properties of the mount.